Byran Uesugi had a Jekyll-Hyde personality – neighbors described him as a pleasant, good-natured guy who raised fish, but cops said he had a reputation as a hothead with an arsenal of weapons.

His father admitted his son’s temper had gotten out of control and he’d enrolled himself in anger-management counseling sessions in recent years.

What triggered the counseling sessions was an incident at work, when Uesugi kicked an elevator door in a fit of anger, his father said.

A 1977 graduate of Roosevelt HS in Honolulu, Uesugi was a gun enthusiast, had been a member of the school rifle club and was considered an expert marksman.

He had 17 guns registered under his name, including pistols, revolvers and 9mm handgun used in yesterday’s massacre – all accumulated over a period of 17 years, cops said.

Police said they discovered an arsenal of weapons stashed in a locked safe at his home.

The arms included semiautomatic shotguns and other rifles.

And Uesugi had wanted to buy more weapons but was turned down for a firearms permit in January 1994.

The permit denial came after he was busted for criminal property damage at his job following an argument with several co-workers, police Sgt. John Kamai said. It was not clear if he was referring to the incident with the elevator.

Uesuge also had a booze problem and had a 1984 conviction for drunken driving, police said.

But neighbors and co-workers said they only knew the suspect as a pleasant and quiet fellow.

“He was a nice, calm, cool guy – you wouldn’t think anything was wrong at all,” a Xerox employee said of his onetime colleague.

“He was a regular man.”

Neighbors said Uesugi seemed happiest when he worked at home in his side business – raising and selling tropical fish and building glass aquariums to sell to local pet stores.

Neighbor Shirley Kimura said Uesugi operated the business from the small garage of the modest home he shared with his dad, Hiro, and brother.

He would spend “very long” hours inside the garage swapping fish from tank to tank to mate them and cutting glass for the aquariums, neighbors said.

So much so that some folks on his street had gently labeled the 5-foot-5, 180-pound suspect a “a bit of an oddball,” said one neighbor who asked not to be identified.

But the fish business was more a labor of love, than a tried-and-true moneymaker – and Uesugi worked at Xerox as a technician for the past 15 years for a regular paycheck and health benefits.

Mayor Jeremy Harris said preliminary indications revealed that there was nothing in Uesugi’s background to show he was capable of such violence.

“It appears he just snapped,” Harris said. “We may never know what was in his mind or why he did such a dastardly thing.”

The suspect’s dad said he was also mystified by the bloodbath.

“I don’t know. He never said anything this morning,” Hiro Uesugi said.